The present invention relates to a method of purifying solvents via filtration. The present invention also relates to a filtration apparatus for carrying out this method.
Liquid solvents are used in many industrial plants and systems for many different purposes. In this connection, the solvent used in the procedure generally becomes dirty or is contaminated with impurities. In order to purify the solvent, these impurities must then be filtered out in a filtration apparatus that uses at least one filter.
One example of the foregoing exists in purification units that are used for chemically cleaning various material or matter, such as for cleaning clothing or other textile materials. With these cleaning units, the solvent that is used as the cleaning agent to loosen the dirt must be filtered or purified in a filtration apparatus in order to prepare the solvent for a further cleaning process.
However, such filtration apparatus per se must be cleaned at least from time to time, in other words, the foreign substances or impurities that are separated off on the filter as a filter cake must be removed, which is often effected by replacement of the filter. However, since not only the filter cake deposited on the filter, but also the filter itself, contain a large amount of solvent residue, the disposal of the filter and the filter residue (impurities separated off as a filter cake) causes significant problems not only for health reasons but also for environmental protection reasons, and in particular not only with regard to the handling of these materials during cleaning of the filtration apparatus, but also with regard to the final storage or disposal of these materials.
With one known cleaning unit for chemically cleaning material, such as clothing or other textile material (German Pat. No. 31 50 015Jost dated July 14, 1983 and correspondingly U.S. Pat. No. 4,483,160--Jost dated Nov. 20, 1984) the cleaning unit includes a solvent circuit, which can be connected to a treatment chamber for the material and includes a filtration apparatus, as well as a warm air circuit that adjoins the treatment chamber, includes an air-conveying mechanism, an air heater, as well as an air cooler, and that serves for removing solvent residue from the material after it has been treated, i.e. cleaned. With this known unit, during the treatment of the material in the warm air circuit, a portion of the air of this warm air circuit is also conveyed through the filtration apparatus in order therein to vaporize solvent residues present in the filter or in the filter cake; these vaporized residues are then condensed at the air cooler of this warm air circuit and are separated out. One of several drawbacks of this heretofore known cleaning unit is that the vaporized solvent residues, which are carried along by the air out of the filtration apparatus, pass into the treatment chamber where the material that is present, for example clothing or other textile materials, then receive more vaporized solvent. However, the chief drawback of this heretofore known cleaning unit is that the warm air that is branched off from the warm air circuit flows through the filter in the filtration apparatus in the same direction in which the solvent also flows through this filter. As a result, the filter cake deposited on the filter is compacted or compressed by the warm air, thereby closing off the passages in the filter cake and in the filter, so that it is impossible to have a uniform flow-through of the filter cake and filter, and it is also impossible to have a rapid and especially complete removal of the solvent residues in the filter and in the filter cake. Another drawback is that for the recovery of the solvent residues in the filter and in the filter cake, operating stages of the cleaning unit are used that are actually provided for a different purpose, namely for removing solvent residues from the material that is to be treated or cleaned, so that these operating stages are not optimally adapted to the conditions necessary for the recovery of solvent residues from the filter and filter cake. This applies, among other things, to the temperature of the warm air stream, which temperature, already for reasons of a careful and protective treatment of materials, for example of delicate textiles, cannot be as high as desired, i.e. cannot be selected in such a way that this temperature suffices for an optimum and complete recovery of the solvent residues in the filter and in the filter cake, especially not in a short period of time.
It is furthermore known (German Offenlegungsschrift 35 12 361Jost dated Aug. 14, 1986), in the filtration apparatus of a cleaning unit for cleaning clothing or other textile material, to provide a heating mechanism for heating the filter as well as the filter cake deposited on this filter. With this heating device, solvent residues in the filter and in the filter cake are vaporized and can be drawn off in this vaporized form. This known arrangement also has the drawback that the filter cake deposited on the filter is compressed and passages in this filter cake and in the filter are closed, since the withdrawal of the vaporized solvent residues is effected in the same direction in which the solvent of the solvent circuit previously flowed through the filter. Furthermore, with this heretofore known arrangement considerable temperature differences in the filter cake cannot be avoided. On the one hand, these temperature differences can, in the hotter regions of the filter cake, cause the impurities that form the filter cake to bake together or to form a connection with the filter that is very difficult to loosen. On the other hand, especially when the impurities that form the filter cake contain fusion-sticky constituents, regions with closed passages in the filter cake and in the filter are formed where low temperatures exist, i.e. where during withdrawal of the vaporized solvent residues cooler air flows through; these regions with closed passages then also make it impossible for the air that flows through during the withdrawal of the vaporized solvent residues to flow uniformly through the filter cake and the filter, thus also making it impossible to completely remove the solvent residues. In addition, if the solvent is a readily inflammable substance, the heating device provided in the filtration apparatus also causes great problems for safety reasons.
It is therefore an object of the present invention to provide a method and apparatus that avoid the aforementioned drawbacks and make it possible to safely and reliably remove solvent residues from the impurities, within the pertaining unit, prior to removal of these impurities from the filtration apparatus and prior to storage and disposal thereof.